In recent years, asynchronous networks such as Internet Protocol (IP) and Asynchronous Transfer Mode (ATM) have become tremendously popular. These networks interconnect various lower layer networks (e.g., Ethernet, SONET, SDH, ADSL, etc.) to provide connectivity between end-points. In particular, the Internet has evolved to a global multi service network through which a great span of different applications communicate (e.g., web browsing, email, telephony, media streaming, and video conferencing).
In asynchronous networks, differentiated forwarding services can be created. While ATM offers multiple forwarding services (e.g., available bit-rate (ABR), constant bit-rate (CBR), etc.), IP networks has traditionally only offered one forwarding service (i.e., the best-effort service). In recent years, formal support for differentiated services has been added to the Internet architecture and Internet Service Providers (ISPs) can thereby configure their IP networks for multiple services.
Configuring an asynchronous network for multiple services, of which some offer guarantees or assurances on the forwarding quality given to users, means to divide forwarding resources such as buffer capacity and forwarding capacity into different classes. We refer to both buffer capacity and forwarding capacity as forwarding recourses. Then, the admission to classes that will carry traffic for which guarantees or assurances shall be given needs to be controlled. We refer to this as admission control.
Note that admission control can be used also to maintain guarantees or assurances in single service networks.
The admission control mechanism can be distributed in network nodes, or centralized in one or more servers or dedicated computers. All these different possibilities are in this text called a node. When it is referred to a node in this text it is thus meant to include both network nodes and servers and dedicated computers. When distributed, the admission control can be invoked by e.g. a signalling protocol. The location of the admission control mechanism does not directly influence the forwarding quality guaranteed or assured for the service in question. Instead the location may affect scalability, performance, etc.
For asynchronous networks, high utilisation of forwarding resources can be achieved through statistical multiplexing. Then, for services offering guarantees or assurances on forwarding quality, the amount of traffic at individual links needs to be carefully controlled.
Knowing the peak-rates of Application Data Flows, ADFs, deterministic forwarding guarantees can be offered through admission control (i.e., sources make an admission request through the network, or to an admission control server before sending any traffic). A peak-rate is the maximum rate at which an ADF can send traffic in a given time interval, see FIG. 1. The average-rate is often calculated over a long time interval, while peak-rate should be calculated over a much shorter time interval.
Unfortunately, offering deterministic guarantees results in low network utilization when ADFs have varying sending rates (e.g., video coders such as ITU-T H.263 produces varying amounts of data depending on movements in the encoded picture). For ADFs having varying sending rates, network utilization can be improved though statistical multiplexing. Several independent flows sharing a common resource are said to benefit from statistical multiplexing if the sum of their peak rates can exceed the total link bandwidth without resulting in quality degradation. This is based on the assumption that the flows send at their peak-rates independently of each other and therefore distributed over time.
To improve network utilization through statistical multiplexing, the sum of peak-rates for ADFs sharing a common link must exceed that link's forwarding capacity, or the portion of that link's capacity allocated for these ADFs. Note, however, that the sum of these ADFs average rates must not exceed that capacity. Then, the link will be overloaded and no forwarding guarantees can be offered.
Since the sum of peak-rates can exceed the (allocated) link capacity, offering deterministic guarantees is not possible. Statistical guarantees on that e.g. the loss-rate at such a link does not exceed a pre-defined value can however be offered. Then, statistical properties of each individual ADF, or the aggregate of all ADFs must be known to calculate the risk of violating the statistical guarantee when accepting an additional ADF for the link in question.
Statistical properties of ADFs might be known beforehand (e.g., they can be given from the definition of the speech codec used), or they can be estimated through measurements. The statistical properties for traffic of e.g. IP telephony applications can be reasonably predictable and the risk of violating the statistical guarantee in question can thus be calculated without measuring these properties.
The statistical properties of ADFs can however be very unpredictable (e.g., for video conference applications the statistical properties of their traffic depends on movements of people participating in the conference). For such applications, measuring these properties is preferable.
The information on statistical properties needs to be accurate for very short time-scales to be used in the mathematical methods used to calculate the risk of violating given guarantees. This means that measurement-based admission control for statistically guaranteed services requires network nodes to perform operations with high time complexity (i.e., processing intensive operations).
Another approach to offer assurances on forwarding quality while allowing for statistical multiplexing is to make admission control decisions using a forwarding resource threshold for each link in a network. That is, a maximum amount of forwarding resources requested from ADFs for each such link, e.g. a maximum sum of ADF bit-rates for each such link. We refer to this threshold as the provisioning level. In the claims it is however called threshold.
The sum of accepted forwarding resources for ADFs plus the forwarding resource of the request to be evaluated can be compared with each link's provisioning level to decide whether one or more of these levels is exceeded or not. An advantage with this approach is that support for advance reservations easily can be included. Such a threshold based admission control is described in “Quality of Service Agents in the Internet. Ph.D. thesis Olov Schelén, August 1998. ISSN 1402-1544. ISRN LTU-DT-98/26-SE”. IQ-Man™ described in this thesis take admission decisions using separate provisioning level for each link in a network.
A problem in this system is that the links are not fully utilised when aggregates of ADFs having different peak and average rates use the links.